Electronic tuning or indicating tube



Filed Nov. 29, 1950 5 Shegts-Sheet 1- y 3 W5 JACOB] ETAL 1 2, 39,398

ELECTRONIC LI'UNING 0R INDICATING TUBE .fizuen/ 0715. Werner Jco 6 y 19, 5 w. JACOB] ETAL 2,639,398 I ELEcTRoNIC TUNING OR INDICATING TUBE Filed Nov. 29, 1950 5 Sheets-Sheet 2 five 0715, Warner Jaob', flfmamizfindarerj Q 5 Sheets-Sheet 3 were W. JACOB! E1" AL ELECTRONIC TUNING OR INDICATING TUBE May 19,1953

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Filed NOV. 29, 1950 5 Sheejis-Shegt 5 y 1953 I w. JACOB! ETAL I 2,639,398

' ELECTRONIC TUNING 0R INDICATING TUBE Patented May 19, 1953 ELEGTRONIC TUNING ORINDICATING TUBE.

Werner Jacobi, Erlangen, Hermann Hinderer, Marl'offstein bei Erl'angen, and. Karl Buclita, Karlsruhe, Germany, assignors'to Siemens &

Halske, Aktiengesel'lschaft, Munich and' Berlin- Siemensstadt, Germany, a German: corporation Application Novemben29, 1950, Serial No. 204,122 In Germany December 9,1949

This invention relates to electronic. tubes-com:- prising luminescent shields or electrodes carrying a luminescent substance which is s'el'ectiveli. affected by" electrons, depending on the" voltage difierential of a control electrode relative to a cathode, and is. particularly concerned with tubes 01;" this kind which are suitable to function. as indicator or'tuning tubes, for example, for wireless receivers or the like.

Indicating or tuning tubes have been proposed. in which the luminescent shield is of. narrow, elongated form, providing for an indication a manner that may be: likened to the indication of. a. thermometer. A control system is insuch structures disposed between the cathode and the luminescent anode, which provides for differential electron action of the anode so that a varia".

bio-area of theshield is caused to luminesce; depending on'the voltage'placed on the control elec trode.

The object of the invention is to facilitate: the legibility or" the indication in such indicating." or tuning: tubes.

One oi the requirements in realizing this. Oh- J'ect is to make the: luminescent shield clearly vis ible within a large angular area wthout requir-- log the. use of transparent or translucent conductive surfaces. Some of the previously proposed structures have the drawback of requiriitig'r observation of the luminescent shield from. the rear so as to. obtain distinct legibility of the iii-mited luminous spot. To overcome this drawback it has been suggested to employ in the proposed structures a. backing for the luminescentshieldl in the form of a glassbody or a wire-netting or" the like.

Another previously proposed structure provides for the viewing. oi the luminescent shield; b

with a very thin threadlike cathode and. disposing such oath-ode. in front of. the luminescent z'hieldin a position in line with the: direction of observation The observation of the luminescentsoield is. either not at all or only negligibly attested the cathode is made. in the form of. an extremely thin band, filament or the like. The

cathode is preferably made in the. for-m of a directly heated cathode, since it forms in such case. merely an extremely thin, threadlike faintly glowing streak whichis hardly perceivable by the by taking into account the thickness. of the fila merit and its insulating layer'as well as the emission layer, and finally the backing for the cruis sion. layer. The heating capacity of: such an im d-irectiy heated. cathode rises with its length. This characteristic, however, is many cases undesired. For example, in. a. wireless A. Con-,0. receiver, in. which. the heating filaments of the various tubes are in series relationships. there isonly a relatively small partial voltage available tor the tuning: tube, thus resulting in predeter-- mined heating requirements therefor; Accord ingiy, it-may happen that the available heating content does not sufiice for a cathode of. relatively considerable length, accordance with the. invention, suchdisadvantage may be overcome, as indicated before, by forming the cathode: adirectly heated cathode.

In accordance with another object, the invenproposes to obtain the heating current for the directly-heated cathode fromthe anode current. supply of. the corresponding circuit of the device. in which the tuning tube is used. In the case of directly heated.v cathodes, the required heating current is smaller than. with indirectly heated. cathodes. It is possible, in accordance with the invention, to dimension the heating filaments so as to permit operation with extraordiharily'low current, thereby making is possible to conduct over the heating filament of the turn ing or indicating tube the anode current of the entire apparatus, or or one or more tube stages thereof. It is iurthermorepossible to providea separate circuit for the heating oi the tuning.

tube and: to include in suchcircuit the field circuit of an electrodynam-ic loud-speaker.

It is particularly advantageous whenusin-g the anode current of only one tube stage to employ for this purpose the: terminal. or final. stage. However, the anode currents of the high-fre .quency and intermediate-frequency tube stages may also be employed. The further advantage is thereby obtained that the entire anode current of the corresponding stage is altered incident to accurate tuning as Well as at the inception of the fading adjustment.

In accordance with another feature of the invention, the arrangement may therefore be such that the variations of the anode current act up the tuning indication in a supporting sense; for,

if the tuning is carried out so that the entire luminescent shield surface becomes luminous in detuned condition or in badly tuned condition of the apparatus, while the luminous area diminishes incident to accurate tuning, such effect will be supported by diminishing emission. The drop of the anode current signifies in such case a decrease in the heating current and therewith decrease of the emission. The exact tuning is thus accentuated.

There are for the indicating or tuning tube made in accordance with the invention numerous other uses aside from its use as a tuning tube in wireless receivers. It is possible, for example, to

equip with such a tuning tube measuring instruments in which bridge circuits have to be tuned relative to one another. The invention may also be used in comparative measurements of voltages and currents, and in the control circuits of amplifiers. Measuring instruments have been usually used heretofore in apparatus for supervising the disconnect control in the case of power amplifiers or the like, employing a suitable measuring device, e. g., a rotary coil device in the anode circuit of a certain stage. The invention furnishes particular advantages, since relatively high anode currents are available in such instances. 1 The anode current varies considerably with the disconnect control, and the different anode currents can therefore be utilized to act in a supporting sense, so far as the indication effect is concerned. This is important, e. g., in the case of B-amplifiers in which the anode current increases with progressive disconnect operation. The operation may therefore be carried out in such a manner that the luminous area increases upon the luminescent shield, with increasing disconnect action. However, the anode current'also diminishes in the case of limited disconnect action, and the emission of the cathode diminishes likewise in accordance with the reduced heating capacity, therefore'permitting again a supporting action, as indicated.

The objects and features outlined above and additional objects and features will be brought out in the course of the detailed description of certain embodiments, which will presently be rendered with reference to the accompanying diagrammatic drawings. In these drawings,

Fig. 1 shows in diagrammatic perspective manner essential elements in the structure of an indicating or tuning tube made in accordance with the invention;

Fig. 2 indicates a schematic section of the electrode arrangement of another embodiment;

. Figs. 3-6 illustrate circuit examples;

Fig. '7 represents diagrammatically the use of the electrode system of one form of the invention in a circuit;

Fig. 8 shows another embodiment of an indicating or tuning tube made in accordance with the invention;

Figs. 9-11 illustrate, in diagrammatic manner, optical aid means for facilitating observation of the indication; and

Fig. 12 represents diagrammatically an embodiment having a luminescent shield of a modified form.

In Fig. l, which shows inschematic manner the essential parts of one embodimentof the new indicating or tuning tube, there is provided, as seen from the front, in the direction of observation, the cathode I which is made in the form of a directly heated threadlike filament. A shutter 2 having a wedge-shaped cutout is used as a control electrode. In back of this control electrode is disposed the luminescent shield or indicating anode 3. The direction of observation is indicated by the arrow 4.

In the embodiment shown in Fig. 2, numeral llagain designates the threadlike cathode. The observation is along the sides of the cathode in the direction of the arrow 4 toward the luminescent shield 3. The intensity of the luminosity or the width of the luminous area is varied by means of the control shutters or grids 5 and 6. The entire emission of the cathode may, in addition, be affected by the control shutter "I.

Fig. 3 shows in schematic representation a basic circuit diagram which indicates the connections essential for the invention, e. g., for

an A. C.-D. C. wireless receiver. The current from a source connected to the terminals 8 and 9 is conducted over the rectifier tube Ill and supplies in usual manner, at the positive terminal ll of the capacitor l2, the required anode potential which is a direct current potential.

The direct current potential is smoothed out by .been omitted. The tube l5 operates as a terminal amplifier tube over the outlet transformer l8 to the loud-speaker IS. The tube [6 may be a high-, intermediateor low-frequency amplifier tube, and the tube I! may be, e. g., a mixing or an oscillator tube. The cathode resistances 20, 2|, 22 disposed between the cathodes of the individual tubes and the terminal 23 are provided for producing in each instance the required grid bias.

The heating filament or cathode 25 of the indicator tube 24 is disposed in the negative branch of the circuit so that all anode and grid currents flow over it. The capacitor 26 does not merely serve for the decoupling, but in addition to its function as a high-frequency bridging of the heating filament 25, it may also serve the purpose of shunting away from the heating filament the switching-in impulse incident to temporary disruption of operation which may attain high values due to the capacitors l2 and M.

Only the cathode current of a single discharge tube stage is conducted over the heating filament 29 of the indicating or tuning tube 30 in the embodiment shown in Fig. 4. The discharge tube is designated by numeral 21. The heating filament 29 is in series with the cathode resistance 28, constituting part of the cathode resistance and, together with the cathode resistance 28, is bridged by the common capacitor 3|.

In Fig. 5 is indicated an embodiment providing a separate cathode or heating circuit for the heating filament 32 or the indicator tube 33 The anode current source is formed by the rectifier tube 34 and the capacitor 35.

' rent flows from the positive anode terminal 36 through the exciter winding 31 of an electrody- The heating cur-- aosaaos 38v... 3!! to the heating filament 32 oi the tuning tube 33.-

Fig. 6 shows a modification oi the arrangement illustrated. in 4. In thev latter figure. the alternating current is conducted fromthe cathode over the capacitor 3| to. the negative terminal, while in Fig. 6 the output alternating current flows overthe heating filament 40 ot the indicator tube M; The cathode resistance 42 oi the discharge tube '43 is separate and is bridged, by a capacitor 44.

This particular arrangement is. believedv to be important, e. g.,. for use with measuringinstruments. disconnect meters and the like. The discharge tube 43 may in such cases be anoutput indicating tube, e. g., a terminalestage tube teedinginto a loud-speaker. It'is. also possible to use in place of such tube two tubes and to operate themin. opposition. As already mentioned, it is possible, by suitably arranging the circuit, to utilize the. variationsof. the: anode.- current' soas; to operatein. asupportin'g sense with respect tothe sensitivity of indication of the: indicator tube.

Aside from. the above discussedexamples, there are numerous other: possibilities for usingthexinvention, as will be realized. upon considering'ithe utilization of the varying anode currents of amplitude -m.odulated. stages. Such stages'are found, insthe: case of high-frequency and, intermediatefrequency amplifiers, e. g.. in multi-channel receivers in which. the modulation usually includes the. mixing tube. The low-frequency mcdulw tionstage is often also: included. If the entireanode current involved iii-the modulation-,in the case of a plurality of modulation stages, is sumciently great, it may be desirable to utilize the corresponding part of the circuit for heating the indicator tube so as to obtain the previously described supporting action affecting the indication.

In accordance. with the invention, the arrangement just. described may bev modified to. further improve the indication. In the case of anindirectly or partially indirectly heated cathode, the heating filament may be so connected that the: positive pole is placed opposite'the least control effect of the'control-system, while the negative pole is disposedv at the point of the greatest control efiect. Such an arrangement results not only in an improvement of the sens-itivityof indication, but also in actionsand'advantages'with regard to different possibilities of use. The luminescence of more or less of the area ofthe luminescent shield in sucli'a case does not depend proportionally on the control voltage; the indicating. action may then be controlled in accordance with other principles. Desired indieating characteristics may be obtained, by suitably dimensioning the operating currents and operating voltages. It is furthermore possibletoiobtain auxiliary dependence of the indication on anadditi'onal measuring value. a

The operating point for the indication may bea favorably placed by suitably choosing the reference: point for the bias of the control electrode, making it possible to obtain good initial or terminal sensitivity for the indication. It should-. also be'consi'dered that variations of the anode current. do not only cause variations: inthc emission,- but also result. inv a differential voltage drop along the heating filament. This action. may be utilized to support the indicatingidffect.

Fig. '1 shows an example of. a corresponding, embodiment. particularly for use as a tuning indicating device for a. Wireless receiver. Gnly ber which may be a cylinder, e. g.,

that portion within-the wireless receiver circuit is indicated which lies normally in. back' of the rectifier and comprises an anode voltage source. The rectified anode voltage is at the terminals -46, the terminal. to being the positive and l6the-negative terminal. Numeral ll indicates a battery which Lethe-anode voltage source. and numeral 48 indicates a capacitor. For the filterim; there is provided a series element, e. g., a choke or resistance 49;. as Well as a; parallel element, e. g., a filter capacitorfiu.

The anode current flows through a plurality of tube stages connectedin parallel, only two discharge tubes 51,. 52 beingshown for the sake v of simplicity of representation. The cathode resistances 53, Marc disposed on the cathode sides, the arrangement being such that the entire oathode current flows over the heating filament 55-01? the indicator tube 56.

The tuning-indicating tube 56, of which the drawing: shows in perspective representation only the electrodes which are essential for the'undenstanding of the invention, comprises a control electrode 51 and. the. luminescent shield 58.. The control electrode 5! is provided. with-ashutterlike cutout. 53 which may be wedge-shaped; A variable control eiiect results in accordancewith the cutout of. the shutter which varies from point to point. The anode 58 forming the luminescent shield is connected with the positive terminal 6! of. the anode voltage over the resistance so, while the negative bias is: connected to-the control electrode 51: in addition tothe control voltages conducted thereto at 62. This negativebias. results from the voltage drop at the resistance 63. The resistance 64, capacitor 85 and the resistance 66 are provided for the filteringand decoupling. The cathode resistances 53, 5.4 are bridged by capacitors 61', 68. Only the capacitor Bil'is disposed in parallel with the re-' sistance' 5'4, but the: capacitor 61' bridges the oath-- ode resistance together with the heating filament, so that the amplified alternating voltages are: conducted tothe discharge tube 5' I over the threadlike cathode or heating filament 55. The arrangement results in an auxiliary control by the alternating voltages. Such a circuit arrangement is of importance, e. g., for the measuring of disconnect operations and the like, comprising circuits in which flow relatively high normal voltages. The normal alternating current in such circuits may thus be utilised for theau-xiliary control of the indication.

Fig; 8 shows an-embodiment of a luminescent shield indicating tube having an opaque outer member. This member may be a coating, layer upon the glass wall of the tube, or a suitable sheet material member drawn over the glass wall. A slotli'keopening may be provided in the" opaque member in order to observe the indication The opening is disposed and dimensioned such a manner that the observation always takes place in identical direction.

Numeral S9 in Fig. 8 indicates the outer memmade of aluminum, surrounding the tube. At the lower end of the tube are the socket pins 85. In order toobserve the indication upon the luminescent shield, there is provided: the narrow, elongated Window opening 70'. The control electrode may be: a. slot-shaped shutter or frame H which is soconstructed as to produce, depending on the control voltages, two luminescent areas which.

move-from. the top and. bottom toward the middie. The cathode 12 which is. again, a in all,

embodiments disclosed herein, a threadlike filament, is disposed in the direction of observation, also as in all other embodiments, in front of the control electrode H and the latter is disposed in front of the luminescent shield 13. The cathode 12 is so thin that it does not obstruct the observation of the indication.

One or more scale calibrations, in accordance with desired division schemes, may be provided marginally of the observation opening. For example, the divisions or gradations may be very fine on one side and in larger increments on the other. Black-white scale gradations are particularly distinct. The scale gradations may be imprinted or etched on the outer cylinder, if desired, in contrasting colors. Two scales I4, are shown in Fig. 8 along the margins of the observation opening 10. They may follow a black-white scheme or may be in contrasting colors, as mentioned before. The scale 14 may have fine subdivisions or gradations and the scale 15 may have marker lines, one to every five or ten of those on the scale M.

It may be desirable in many instances to carry out the observation with a greater viewing angle. Optical means may be used to emphasize subjective luminosity and to magnify the dimensions of the indication image. For example, an optical magnifying lens may be disposed in front of the indicating tube. Such a lens will magnify the impression of luminosity and, if optically of good quality, will permit to reach the limits of legibility. The lens may be mounted with a tube or may form part thereof. It may also be made in the form of a cylindrical lens. The optical magnification of the length of the luminous indication image is in such case of secondary importance as compared with the brightness and distinct legibility thereof. The indication may be widened by the use of a cylindrical lens. If the luminous spot produced by the control shutter terminates at its margins in a thin point, such point may be widened for viewing, by the use of a lens. The borderline between bright and dark zones can thus be brought into view and emphasized.

Numeral 16 in Fig. 9 indicates the threadlike cathode disposed in front of the control electrode 11, which is made in the form of a shutter having a wedge-shaped or trapezoidal cutout. In back of the control electrode is the luminescent shield anode 18 having a narrow, elongated shape. The various elements of the electrode system are thus similar to those described in connection with the previous embodiments and are similarly disposed. In front of this electrode system is provided a cylindrical lens 19 which operates in the manner of a watchmakers magnifying glass. The lens does not produce subjective magnification of the indicating image, but an apparent widening of the terminal points which, as such, are bright and sharply outlined but terminate in thin ends, thus making these points to appear as wide beams.

Other optical means may be used to widen the pointed luminous image to form a broad band of uniform width and brightness. It is desirable to use for this purpose an optical system which causes such an apparent deformation of the luminous image that its momentary position (depending on the control voltage) is either not at all affected or is affected only negligibly, while its width is reproduced in a plurality of astigmatic, preferably overlapping, images. In

8. order to-' obtain such effect, there may be pro vided a system of lenses in place of the single cylindrical lens of Fig'. 9. For example, a plurality of lenses disposed side by side may be used. It is also possible to use a transparent body provided with a plurality of grooves or ridges extending in parallel relationship. In order to produce as bright an image as possible, the multiple lens may be combined with a cylindrical lens operating as a magnifier. This may be carried out in simple manner by grinding the underside of a thick cylindrical'lens-in a flat plane and forming thereon a plurality of grooves or ridges which act in the manner of a multiple cylindrical lens.

Fig. 10 shows an embodiment of such a structure in a view looking upon the indicating system from above, such system comprising the previously described elements including the thfireadlike cathode 80, the double-wedged or double-trapezoidal shutterlike control electrode 8| disposed in the back'of the cathode, and the luminescent shield anode 82 disposed in back of the electrode 8|. The control electrode is constructed so that luminous points are produced at the two outer ends which move toward one another more or less symmetrically, depending on the control voltage applied. In front of the electrodes is disposed the optical system 83 comprising a thick body the upper side of which is formed in the manner of a cylindrical lens. On the fiat underside-are provided equally spaced parallel grooves or ridges to produce the effect of a multiple cylindrical lens. The optical system may be suitably arranged outside or inside of the discharge tube or may form part of the wall thereof.

A distortion in which the position of the luminous image is not affected at all, or only negligibly so, while its width is magnified, may be produced by employing a plurality of transparent plates disposed in parallel relationship, taking advantage of the fact known that light striking a glass plate at one of its edges will by total reflection radiate outwardly substantially only at the opposite edge. It is not necessary that the light strikes the edge perpendicular to its plane; it may strike at an angle thereto. If a plurality of such glass plates are assembled in a pack, light striking the edges of the pack at one side can then be observed at the edges of all the glass plates at the opposite side.- The light thus will appear as a correspondingly wide image picture. Such a structure appears in Fig. 11.

The electrode system 'shown in Fig. 11 corresponds to the previously described structure. It comprises the threadlike cathode 84, the shutterlike control electrode 85 and the luminescent shield 86. The direction of viewing is indicated by the arrows 81, 88, 89. The picture of the luminous images will appear visible at the edge of each of the glass plates included in the. pack 90, and the succession of the individual images will be recognized as a wide luminous band.

An increase of the brightness of the image picture may also be obtained by the embodiment diagrammatically shown in Fig. 12 which comprises again a threadlike cathode 9| and in back'thereof the shutterlike control electrode 92. Through the cutout in this control electrode is observed the luminous shield 93. The principal direction of viewing is in the direction of the arrows 94, and 96, but it may also be at an angle, as indicated by the arrows 91 and 98. The shutter opening is in practice narrower than shown in the figure, i. e., narrower as compared with its longitudinal extent, which is also true of the previously described figures. The line of observation of the luminescent shield may vary along the longitudinal extent of the cutout; if viewed in a direction perpendicular to the cutout, it will of course be limited by the configuration thereof. The luminescent shield may be so constructed, in view of the fact that it is viewed in a definite principal direction, that it permits an increased reflection for a certain angle of viewing or for a plurality of angles lying in a predetermined plane. A somewhat reduced brightness may then be tolerated incident to viewing the shield from other points of observation, and especially in a direction perpendicular to such plane. The backing for the luminescent substance, which advantageously may be silver-coated, may for this purpose be roughened by transverse grooves 199 extending in a direction which is substantially perpendicular to the principal plane or planes of the angles at which the viewing takes place. Such roughening of the backing of the luminescent shield also results in the additional advantage of increasing the strength of adhesion of the luminescent substance. It is therefore possible to employ backings, e.

silvered surfaces, which, as such, are rather smooth and which otherwise would introduce some hazards so far as adhesion of the luminescent substance is concerned. The parallel roughened grooves may be provided in suitable manner, e. g., by scratching the finished backing body. However, it is also possible to form the grooves prior to finishing the shield body and to thereafter provide the backing with an optically reflecting coacting, e. g., a silver coating.

Examples of previously proposed structures mentioned in the preamble may be had by considering co-pending applications Ser. Nos. 188,161 and 188,162, filed October 3, 1950, now Patents Nos. 2,594,025 and 2,594,026.

Changes may be made within the spirit of the appended claims.

We claim:

1. An electronic tube, especially for use as a tuning or indicating tube in an electronic circuit, comprising a plurality of electrodes which are structurally formed as specified and which are as seen in the direction of observation by an observer disposed as specified, namely, first, an elongated threadlike cathode, second, a flat control electrode in which is formed a cutout, said control electrode being disposed in back of said cathode substantially in parallel therewith and in its entirety radially spaced therefrom, and third, an elongated generally fiat and narrow luminescent anode disposed in back of said control electrode substantially in parallel therewith and rearwardly spaced therefrom.

2. The structure defined in claim 1, together with auxiliary electrode means for affecting the luminous image produced on said luminescent anode.

3. The structure defined in claim 1, together with opaque outer cover means for said tube, and a slotlike cutout in said outer cover means for observing the luminous indicating action of said luminescent anode inside said tube.

4. The structure defined in claim 1, together with opaque outer cover means for said tube,

scope and 19 a slotlike cutout in said outer cover means for observing the luminous indicating action of said luminescent anode inside said tube, and scale means disposed marginally of said cutout.

5. The structure defined in claim 1, together with optical means disposed in front of said cathode for magnifying the width of the luminous indicating image on said luminescent anode.

6. The structure defined in claim 1, together with optical means disposed in front of said cathode for magnifying the width of the luminous indicating image on said luminescent anode, said optical means being disposed in front of said tube facing said cathode in line with the direction of observation.

7. The structure defined in claim 1, together with optical means disposed in front of said cathode for magnifying the width of the luminous indicating image on said luminescent anode, said optical means forming part of said tube.

8. The structure defined in claim 1, together with a cylinder lens disposed in front of said cathode for magnifying the width of the luminous indicating image on said luminescent anode.

9. The structure defined in claim 1, together with optical means disposed in front of said cathode for magnifying the width of the luminous indicating image on said luminescent anode, said optical means being constructed so as to cause subjective deformation of the width of the luminous image by dissolving such image optically into a plurality of astigmatic preferably overlapping images.

10. The structure defined in claim 1, together with optical means disposed in front of said cathode for magnifying the width of the luminous indicating image on said luminescent anode, said optical means comprising a transparent body provided with a plurality of parallel ridges.

11. The structure defined in claim 1, together with optical means disposed in front of said cathode for magnifying the width of the luminous indicating image on said luminescent anode, said optical means comprising a plurality of transparent pl-atelike members disposed side by side.

12. The structure defined in claim 1, wherein said luminescent anode comprises a reflecting backing carrying a luminescent substance, said backing being roughened by ridges disposed thereon in a plane which extends substantially perpendicular to the direction in which said anode is viewed for observing the luminous indication thereon.

13. The structure defined in claim 1, wherein said luminescent anode comprises an optically reflecting backing carrying a luminescent substance, said backing being roughened by ridges disposed thereon at an angle to the longitudinal axis of the shield.

WERNER JACOBI. HERMANN I-IINDERER. KARL BUCHTA.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,994,726 Orth Mar. 19, 1935 2,334,473 Carlson Nov. 16, 1943 2,366,320 Elston Jan. 2, 1945 

